Catalysts For Low-temperature CO Oxidation Research Articles

Hydrogen spillover enhanced hydroxyl formation and catalytic activity toward CO oxidation at the metal/oxide interface.

H2-promoted catalytic activity of oxide-supported metal catalysts in low-temperature CO oxidation is of great interest but its origin remains unknown. Employing an FeO(111)/Pt(111) inverse model catalyst, we herewith report direct experimental evidence for the spillover of H(a) adatoms on the Pt surface formed by H2 dissociation to the Pt-FeO interface to form hydroxyl groups that facilely oxidize CO(a) on the neighboring Pt surface to produce CO2. Hydroxyl groups and coadsorbed water play a crucial role in the occurrence of hydrogen spillover. These results unambiguously identify the occurrence of hydrogen spillover from the metal surface to the noble metal/metal oxide interface and the resultant enhanced catalytic activity of the metal/oxide interface in low-temperature CO oxidation, which provides a molecular-level understanding of both H2-promoted catalytic activity of metal/oxide ensembles in low-temperature CO oxidation and hydrogen spillover.

Nanoscale Co-based catalysts for low-temperature CO oxidation

A novel MOF-derived metallic cobalt-based catalyst for low-temperature CO oxidation was developed. This catalyst exhibited high catalytic activity even at a temperature as low as −30 °C and improved tolerance of moisture as compared to other Co-based materials.

Synthesis, Characterization and Application of Co–MgO Mixed Oxides in Oxidation of Carbon Monoxide

The present work deals with the synthesis of nanostructured Co–MgO mixed oxides with different weight ratios of cobalt by a facile co-precipitation method as a catalyst for low-temperature CO oxidation. The prepared samples were characterized by X-ray diffraction (XRD), N2 adsorption/desorption (BET), Fourier transform infrared spectroscopy (FTIR), and transmission and scanning electron microscopies (TEM and SEM) techniques. The results revealed that inexpensive cobalt–magnesium mixed metal oxide nanoparticles have a high potential as catalyst in low-temperature CO oxidation. The Co–MgO mixed oxide with 30 wt.% cobalt had the highest activity. The results showed that the catalysts pretreated under O2-containing atmosphere possessed higher activity compared to the catalyst pretreated under H2 atmosphere. Co–MgO catalyst showed a good repeatability in reaction condition. The stability test exhibited that the Co–MgO mixed oxides were highly stable for CO oxidation over a 30 h time on stream in the feed gas containing a high amount of moisture and CO2.

High Activity of Gold/Tin-Dioxide Catalysts for Low-Temperature CO Oxidation: Application of a Reducible Metal Oxide to a Catalyst Support

Au/SnO2 catalysts were prepared by both the solid grinding (SG) method and the deposition precipitation (DP) method. Compared to samples prepared by the DP method, the SG samples showed remarkably high activity for low-temperature CO oxidation, because of a more efficient Au deposition on SnO2 by the SG method. Detailed analysis revealed that the perimeter interface between Au and SnO2 is the active site for catalysis. The present results suggest that the activity of Au/oxide catalysts is associated with the reducibility of the metal oxide supports at the perimeter interface.

Alumina hollow microspheres supported gold catalysts for low-temperature CO oxidation: effect of the pretreatment atmospheres on the catalytic activity and stability

Hierarchically organized γ-Al2O3 hollow microspheres were prepared via a hydrothermal method using potassium aluminum sulfate and urea as reactants. The corresponding Au/Al2O3 catalysts were obtained using a deposition-precipitation (DP) method. The effect of the pretreatment under different atmospheres (N2, air, and H2) on the activity and stability of the Au/Al2O3 catalysts in CO oxidation was investigated. The results showed that the pretreatment under H2 atmosphere improved the low-temperature CO oxidation activity. Furthermore, a 50 h long-term test at 30 °C showed no significant deactivation for the H2-pretreated catalyst. Moreover, the catalytic activity was promoted by H2O vapor in all cases, and the H2-pretreated catalyst exhibited a good tolerance in the co-presence of CO2 and H2O. Finally, oxygen temperature-programmed desorption (O2-TPD) and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) revealed that the reductive atmosphere pretreatment greatly improved the CO adsorption capacity and facilitated the oxygen activation.

Design of a highly active Pt/Al2O3catalyst for low-temperature CO oxidation

A series of Al2O3 supported platinum catalysts (Pt/Al2O3), were prepared by a colloid deposition route. The Pt chemical state and nanostructure over the Pt/Al2O3 catalysts were characterized after calcination treatment using X-ray photoelectron spectroscopy and transmission electron microscopy. The Pt chemical state and nanostructure depended on the treatment temperature: a metallic Pt surface was formed on the Pt/Al2O3 catalysts calcined at low temperature, whereas oxidized Pt existed after the relatively high temperature treatment. The Al2O3 support acted as an anchor and inhibited the sintering of Pt particles on the catalyst surface through intimate interaction between Al2O3 and Pt. The Pt/Al2O3 catalyst pre-treated at 200 °C (i.e., Pt/Al2O3-200) exhibited relatively high activity for CO oxidation. According to the results of catalyst characterization, Pt/Al2O3-200 could efficiently govern O2 adsorption by trapping O2 molecules on Pt sites and producing active oxygen species. That is, the surface metallic Pt could facilitate the adsorption and activation of O2 molecules even with the CO pre-adsorption on the surface of Pt particles.

Active gold-ceria and gold-ceria/titania catalysts for CO oxidation: From single-crystal model catalysts to powder catalysts

CO oxidation was studied on model and powder catalysts of Au-CeO2 and Au-CeOx/TiO2. Phenomena observed in Au-CeO2(111) and Au-CeO2/TiO2(110) provided useful concepts for designing and preparing highly active and stable Au-CeOx/TiO2 powder catalysts for CO oxidation. Small particles of Au dispersed on CeO2(111) displayed high catalytic activity, making Au-CeO2(111) a better CO oxidation catalyst than Au-TiO2(110) or Au-MgO(100). An excellent support for gold was found after depositing nanoparticles of ceria on TiO2(110). The CeOx nanoparticles act as nucleation centers for gold, improving dispersion of the supported metal and helping in the creation of reaction sites efficient for the adsorption of CO and the dissociation of the O2 molecule. High-surface area catalysts were prepared by depositing gold on ceria nanorods and CeOx/TiO2 powders. The samples were tested for the low-temperature (10–70°C) oxygen-rich (1%CO/4%O2/He) CO oxidation reaction after pre-oxidation (20%O2/He, 300°C) and pre-reduction (5%H2/He, 300°C) treatments. Synchrotron-based operando X-ray diffraction (XRD) and X-ray absorption (XAS) spectroscopy were used to study the Au-CeO2 and Au-CeOx/TiO2 catalysts under reaction conditions. Our operando findings indicate that the most active phase of these catalysts for low-temperature CO oxidation consist of small particles of metallic Au dispersed on CeO2 or CeOx/TiO2.

Mesoporous iron oxide-silica supported gold catalysts for low-temperature CO oxidation

Mesoporous iron oxide-silica composite with a high silica content was synthesized by hydrothermal method, and another composite material with a high iron content was obtained by etching part of silica in alkaline solution. Gold catalysts were loaded onto both composites by a deposition-precipitation method, and used for CO oxidation. The samples were characterized by Brumauer-Emmet-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), transmission electron microscope (TEM) and scanning electron microscope (SEM) techniques. Both composites had high specific surface areas and were amorphous. The Au nanoparticles dispersed on the surface of the composites existed in metallic state. Composite with high silica content was not suitable for Au loading, and its supported gold catalyst showed poor performance in catalytic reaction. In contrast, composite with high iron content allowed efficient Au loading, and CO could be oxidized completely at low temperature on its supported gold catalyst. The effects of deposition-precipitation pH values on Au loading and activity of the catalyst were investigated, and the results indicated that Au loading was the highest and the catalyst was the most active for CO oxidation when the synthesis pH was adjusted to 8.

Fabrication and catalytic properties of Pd and Ce decorated carbon nanotube-TiO2 composite catalysts for low-temperature CO oxidation

In this paper, a series of Pd-Ce/acid-treated carbon nanotube (TCNT)-TiO2 composite catalysts were designed and fabricated carefully for low temperature CO oxidation. After covering of TiO2 on the surface of CNT, the catalytic activity enhanced obviously because of the synergetic effect of CNT and TiO2. The composite material CNT-TiO2 was more suitable for the catalytic support than only TCNT or TiO2. The experimental conditions, such as the ratio of CNT/TiO2, the prepared method of Pd nanoparticles, the effect of the Ce element, etc., were investigated in detail. The final result showed that Pd-Ce/TCNT-TiO2 catalyst prepared by NH3 coordination impregnation method when TCNT:TiO2=12:88–24:76 had superior activity on low temperature CO oxidation. The high dispersion of Pd nanoparticles and enriched surface Pd species on the Pd-Ce/CNT-TiO2 catalyst prepared by NH3 coordination impregnation method was the two main reasons of obtaining higher catalytic performance. By adjusting the research conditions, the CO molecules could be complete oxidized at about 35°C, which was lower than many catalysts reported in the existed literature.

Graphdiyne as a metal-free catalyst for low-temperature CO oxidation

The oxidation of CO has attracted great interest in recent years because of its important role in enhancing the catalyst durability in fuel cells and in solving the growing environmental problems caused by CO emission. The usually used noble metal nanocatalysts are costly and require high reaction temperature for efficient operation. We report here a density functional theory (DFT) study of low-temperature CO oxidation catalyzed by graphdiyne, which is a new two-dimensional periodic carbon allotrope with a one-atom-thick sheet of carbon building of sp- and sp(2)-hybridized carbon atoms and has been shown in our recent work to have high catalytic activity for oxygen reduction reactions (ORRs). We studied the adsorption properties of CO and O2 on graphdiyne, simulated the reaction mechanism of CO oxidation involving graphdiyne, and analyzed electronic structures at each step of reaction progress. The simulation results indicate that the adsorption of O2 prevails over CO adsorption on the graphdiyne sheet; the reaction of CO oxidation by adsorbed O2 on graphdiyne proceeds via the Eley-Rideal (ER) mechanism with a decrease in the energy of the system and the energy barrier as low as 0.18 eV in the rate-limiting step. The oxidation reaction includes the breakage of the O-O bond in the adsorbed O2, formation of the metastable carbonate-like intermediate state, and the creation of CO2 molecules. The results presented here demonstrate that graphdiyne is a good, low-cost, and metal-free catalyst for low-temperature CO oxidation, can be used to solve problems caused by environmental CO emission and has a high ability of CO tolerance by its removal through oxidation in fuel cells.

CO oxidation catalyzed by Pt-embedded graphene: a first-principles investigation.

We addressed the potential catalytic role of Pt-embedded graphene in CO oxidation by first-principles-based calculations. We showed that the combination of highly reactive Pt atoms and defects over graphene makes the Pt-embedded graphene a superior mono-dispersed atomic catalyst for CO oxidation. The binding energy of a single Pt atom onto monovacancy defects is up to -7.10 eV, which not only ensures the high stability of the embedded Pt atom, but also vigorously excludes the possibility of diffusion and aggregation of embedded Pt atoms. This strong interfacial interaction also tunes the energy level of Pt-d states for the activation of O2, and promotes the formation and dissociation of the peroxide-like intermediate. The catalytic cycle of CO oxidation is initiated through the Langmuir-Hinshelwood mechanism, with the formation of a peroxide-like intermediate by the coadsorbed CO and O2, by the dissociation of which the CO2 molecule and an adsorbed O atom are formed. Then, another gaseous CO will react with the remnant O atom and make the embedded Pt atom available for the subsequent reaction. The calculated energy barriers for the formation and dissociation of the peroxide-like intermediate are as low as 0.33 and 0.15 eV, respectively, while that for the regeneration of the embedded Pt atom is 0.46 eV, indicating the potential high catalytic performance of Pt-embedded graphene for low temperature CO oxidation.

Highly active PdCeOx composite catalysts for low-temperature CO oxidation, prepared by plasma-arc synthesis

The plasma-arc method was adapted for the synthesis of composite palladium–ceria catalysts, which were utilized for the low-temperature oxidation of carbon monoxide. The Pd/CeO2 catalysts were synthesized in two steps: step 1 – direct synthesis of palladium–cerium–carbon composite PdCeC in the plasma-arc chamber, and step 2 – calcination of the composite with carbon burnout in air at 600, 700, and 800°C. Catalytic testing using temperature-programmed reaction demonstrated the high efficiency of the synthesized catalysts during the oxidation of carbon monoxide and their ability to oxidize CO at temperatures as low as room temperature. In comparison with a catalyst that had similar morphology and palladium content but was prepared by coprecipitation, the synthesized catalyst showed that the calculated TOF value for the composite catalyst was two–three times higher than that of the catalyst prepared by chemical methods.A variety of physical methods (HRTEM, XRD, XPS, Raman spectroscopy and others) were used to examine the microstructure, composition and electronic state of the composite components in detail after the high-temperature calcination stage of catalyst synthesis. It was shown that high catalytic activity was provided by the formation of a high-defect fluorite structure of PdCeOx solid solution and highly dispersed PdOx nanoparticles. The XPS data suggest that carbon nanostructures mixed with palladium and cerium in the initial PdCeC composite prevent sintering during high-temperature calcination and play a key role in the formation of a high-defect solid solution of palladium in the CeO2 structure, which has a high concentration of Ce3+ and oxygen vacancies.

Morphologic effects of nano CeO2–TiO2 on the performance of Au/CeO2–TiO2 catalysts in low-temperature CO oxidation

CeO2–TiO2 composite nanorods and nanoparticles were synthesized by hydrothermal and co-precipitation methods, respectively; with the CeO2–TiO2 composite of different morphologies as supports, Au/CeO2–TiO2 catalysts were prepared by the colloidal deposition method and used in CO oxidation at ambient temperature. N2 sorption, XRD, HRTEM, H2-TPR, XPS and XAS characterizations were used to clarify the relationship between the morphologic properties of CeO2–TiO2 and the catalytic performance of Au/CeO2–TiO2. HRTEM results show that Au/CeO2–TiO2 nanorods are dominated by CeO2 nanocrystals with (110) and (100) facets, while the Au/CeO2–TiO2 nanoparticles mainly expose (111) planes. XPS and XAS results reveal that both Au/CeO2–TiO2 nanorods and Au/CeO2–TiO2 nanoparticles are similar in the state of gold species. H2-TPR results indicate that the presence of Au strongly promotes the reduction of CeO2 in the Au/CeO2–TiO2 nanorods. The catalytic activity of Au/CeO2–TiO2 in CO oxidation is strongly related to the morphology of CeO2–TiO2; the CeO2–TiO2 nanorods as support endow the Au/CeO2–TiO2 catalyst with much higher activity than the CeO2–TiO2 nanoparticles. The (110) and (100) facets of CeO2 dominated in the Au/CeO2–TiO2 nanorods are able to promote the oxygen migration and gold dispersion, which may then evidently enhance the redox capacity and catalytic activity of the Au/CeO2–TiO2 nanorods in CO oxidation at low temperature.

Promotion effect of adsorbed water/OH on the catalytic performance of Ag/activated carbon catalysts for CO preferential oxidation in excess H2

Activated carbon (AC) supported silver catalysts were prepared by incipient wetness impregnation method and their catalytic performance for CO preferential oxidation (PROX) in excess H2 was evaluated. Ag/AC catalysts, after reduction in H2 at low temperatures (≤200 °C) following heat treatment in He at 200 °C (He200H200), exhibited the best catalytic properties. Temperature-programmed desorption (TPD), X-ray diffraction (XRD) and temperature-programmed reduction (TPR) results indicated that silver oxides were produced during heat treatment in He at 200 °C which were reduced to metal silver nanoparticles in H2 at low temperatures (≤200 °C), simultaneously generating the adsorbed water/OH. CO conversion was enhanced 40% after water treatment following heat treatment in He at 600 °C. These results imply that the metal silver nanoparticles are the active species and the adsorbed water/OH has noticeable promotion effects on CO oxidation. However, the promotion effect is still limited compared to gold catalysts under the similar conditions, which may be the reason of low selectivity to CO oxidation in PROX over silver catalysts. The reported Ag/AC-S-He catalyst after He200H200 treatment displayed similar PROX of CO reaction properties to Ag/SiO2. This means that Ag/AC catalyst is also an efficient low-temperature CO oxidation catalyst.

Intermetallic NaAu2 as a Heterogeneous Catalyst for Low-Temperature CO Oxidation

The enhanced stability and modified electronic structure of intermetallic compounds provide discovery of superior catalysts for chemical conversions with high activity, selectivity, and stability. We find that the intermetallic NaAu2 is an active catalyst for CO oxidation at low temperatures. From density functional theory calculations, a reaction mechanism is suggested to explain the observed low reaction barrier of CO oxidation by NaAu2, in which a CO molecule reacts directly with an adsorbed O2 to form an OOCO* intermediate. The presence of surface Na increases the binding energy of O2 and decreases the energy barrier of the transition states.

Origin of the high activity of Au/FeOx for low-temperature CO oxidation: Direct evidence for a redox mechanism

FeOx-supported gold nanocatalyst is one of the most active catalysts for low-temperature CO oxidation. However, the origin of the high activity is still in debate. In this work, using a combination of surface-sensitive in situ FT-IR, Raman spectroscopy, and microcalorimetry, we provide unambiguous evidence that the surface lattice oxygen of the FeOx support participates directly in the low-temperature CO oxidation, and the reaction proceeds mainly through a redox mechanism. Both the presence of gold and the ferrihydrite nature of the FeOx support promote the redox activity greatly. Calcination treatment has a detrimental effect on the redox activity of the Au/FeOx, which in turn decreases greatly the activity for low-temperature CO oxidation. The gold-assisted redox mechanism was also extended to other metal-supported FeOx catalysts, demonstrating the key role of the FeOx support in catalyzing the CO oxidation reaction.

The role of iron oxide in the highly effective Fe-modified Co3O4 catalyst for low-temperature CO oxidation

Download options Please wait... Article information DOI https://doi.org/10.1039/C3RA41272E Article type Paper Submitted 21 Mar 2013 Accepted 10 May 2013 First published 16 May 2013 Download Citation RSC Adv., 2013,3, 12409-12416 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions The role of iron oxide in the highly effective Fe-modified Co3O4 catalyst for low-temperature CO oxidation J. Li, G. Lu, G. Wu, D. Mao, Y. Guo, Y. Wang and Y. Guo, RSC Adv., 2013, 3, 12409 DOI: 10.1039/C3RA41272E To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Jie Li Guanzhong Lu Guisheng Wu Dongsen Mao Yanglong Guo Yanqin Wang Yun Guo

Study of cupric oxide nanopowders as efficient catalysts for low-temperature CO oxidation

CuO nanopowders were prepared by precipitation from alkaline solutions and were studied by TPR-CO+O2, XRD, TGA, TPD-He and XPS. All of the precipitated samples were characterized by excellent catalytic properties toward the low-temperature (LT) oxidation of CO with similar T50 values of 110°C. In contrast, bulk CuO oxides with sizes greater than 450nm exhibited no activity at low temperatures. Several monolayers of chemisorbed species, such as water/hydroxyls and carbonate/hydrocarbonates, were typically observed at the surface of the nanopowders. These species were not critical for the LT oxidation of CO, and their preliminary removal did not substantially change the activity of the nanopowders. XPS results indicated a high deficiency of the oxygen sublattice of the CuO1−x (x=0.1–0.15) nanopowders, whereas, for the lattice of bulk CuO, the Cu/O ratio was 1. The highly deficient oxygen sublattice resulted in a disproportionation process, which, in turn, resulted in two observed oxygen forms. An oxygen form with Eb(O1s)=531.3eV that is highly reactive toward CO was proposed to be responsible for the high catalytic activity of the CuO nanopowders. Slight differences in the Cu2p shake-up satellite structures were observed between the bulk and nanosized samples, which indicated that the electronic structure in the cationic sublattice had changed.

Study of Fe–Co mixed metal oxide nanoparticles in the catalytic low-temperature CO oxidation

Iron–cobalt mixed metal oxide nanoparticles (Co/Fe molar ratio: 1/5) have been prepared by a simple co-precipitation method and employed as catalyst in low-temperature CO oxidation. The prepared catalysts were characterized by thermal gravimetric and differential thermal gravimetric analyses (TGA/DTG), X-ray diffraction (XRD), temperature programmed reduction (TPR), N2 adsorption (BET) and transmission electron microscopy (TEM) techniques. The results revealed that inexpensive iron–cobalt mixed metal oxide nanoparticles have a high potential as catalyst in low temperature CO oxidation. The results showed that increasing in calcination temperature increased the crystallite and particle size and decreased the specific surface area, which caused a decrease in catalytic activity of prepared catalysts. In addition, the pretreatment conditions affect the catalytic activity and catalyst pretreated under oxidative atmosphere showed the higher activity than those pretreated under reductive and inert atmospheres.

Ag nanoparticles supported on wormhole HMS material as catalysts for CO oxidation: Effects of preparation methods

Ag nanoparticles supported on wormhole HMS material were prepared by in-situ reduction methods (post-assembly and in-situ incorporation methods). The catalysts were characterized by XRD, N2 adsorption–desorption, ICP-AES and TEM. It was found that the preparation methods strongly affected the structure of HMS and Ag particle size. Due to the protection of DDA in the reduction process, Ag nanoparticles with smallest size (4.5 nm) were obtained in in-situ incorporation method (TEOS was added into the mixture solution in the last procedure). Meanwhile, HCHO in the solution can react with DDA, resulting in the decrease of structural order and BET value of the sample. In post-assembly methods, the H-bonding between DDA and the inorganic Si species weakens the affinity of DDA for Ag species, resulting in the aggregation of Ag nanoparticles in the following reduction process. The catalytic activity of catalysts for low-temperature CO oxidation was studied here. Besides the size effect of Ag nanoparticles, it was believed that the structural order of the samples was also crucial for high catalytic activity.